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A weak rotationally resolved absorption band of 12 C 18 O has been identified from photographic VUV spectra and assigned to the k 3 Π ( v = 3) ← X 1 Σ + ( v = 0) forbidden transition. The experimentally determined band origin and upper state rotational constant are in close agreement to that derived from isotopic scaling. A consideration of the intensity structure of the band suggests that the band gains its intensity mainly through a k 3 Π ( v = 3)–E 1 Π ( v = 0) interaction. This is the first direct observation of the k 3 Π state for 12 C 18 O.

The forbidden c3Π–X1Σ+ (0–0) absorption band of carbon monoxide has been reinvestigated under different pressure conditions using the 10.7-m VUV spectrograph at Meudon. Overlap with the allowed C1Π–X1Σ+ (0–0) band at lower transition energy has been taken into account. We have identified a new rotational branch corresponding to an S-type branch and extended the analysis to both higher and lower J. An analysis of the band structure and the low J transition lines suggests that the band gains its intensity predominantly as a result of an interaction of the c3Π state with a 1Σ+ state, most likely the C1Σ+ (v = 0) state. Molecular constants have been obtained for the c3Π state that are in reasonable agreement with those previously published. The apparently anomalous small value for the centrifugal distortion constant is explained by a homogeneous perturbation with the k3Π valence state.

The high-resolution VUV k 3 Π( v = 2, 3, 5) ← X 1 Σ + ( v = 0) "forbidden" absorption bands of CO have been observed and rotationally analyzed in the wavenumber range 90 000-98 000 cm −1 , confirming the recent 3 Π assignment for the upper valence state given by Baker et al. ( Chem. Phys. 178 , 569-579 (1993)). It is also shown that the k 3 Π 2 ( v = 3) spin-orbit component is responsible for the indirect predissociation at J = 31 in the E 1 Π( v = 0) state, while the k 3 π 0 ( v = 5) spin-orbit component is responsible for the indirect predissociation at J = 7 in the E 1 Π( v = 1) state.

Line positions and emission probabilities of the Lyman and Werner band systems of molecular hydrogen have been calculated by solving a system of four coupled Schrodinger equations. We have been able to identify several thousand new lines emitted by a low pressure discharge in H2 from 172 nm down to 78 nm. Many high-J energy levels are reported for the first time. Molecular constants are derived for the (unperturbed) 1Π−u states. Several examples demonstrate that the emission spectrum is very significantly different from that which could be deduced from a knowledge of only the absorption spectrum.

The absorption spectrum of SiF has been observed down to the LiF absorption limit. Rydberg series nsσ npσ , and npπ have been observed up to n = 15, as well as two superexcited states.

The vacuum ultraviolet emission spectrum of molecular nitrogen, produced by a low pressure discharge, is investigated at high spectral resolution from 107.7 to 124.2nm. Band origins are reported for the 103 bands rotationally analyzed.

The vacuum ultraviolet band spectrum system B1Σ+-X1Σ+ of 12C16O, 13C16O, 12C18O, and 13C18O has been extensively investigated both in absorption and in emission with the Meudon Observatory national 10-m vacuum spectrograph. A band lying at 109.9 nm in 12C16O is observed for all isotopes but only in absorption. It is shown to be the B-X(2-0) band. Molecular parameters of the B(v = 0, 1, 2) levels have been derived from simultaneous least-squares fits of the absorption and emission data pertaining to each of these vibrational levels. In addition, a simultaneous fit of the data for the different isotopes provided mass independent Dunham coefficients and equilibrium constants for the B state. Our values obtained for ωe and ωexe confirm the recent determinations which are in disagreement with the values tabulated in standard references. The emission bands exhibit sudden weakenings typical of predissociation. This predissociation further manifests itself in the v = 2 level by a conspicuous broadening of the absorption lines. It may also be the cause of the displacement and anomalous B values observed for this level. From the Dunham coefficients, the following molecular constants (in cm−1 except for re) for the B state have been derived: T e =86926.9 r 3 =1.1197 A . ω e ω e x e B e α e 10 6 D e 12C16O 2161.75 39.84 1.9613 0.0262 6.5 13C16O 2113.56 38.08 1.8749 0.0245 6.0 12C18O 2109.53 37.93 1.8677 0.0244 5.9 13C18O 2060.12 36.18 1.7812 0.0227 5.4 14C16O 2071.52 36.58 1.8010 0.0231 5.5

Numerous (0-v″) bands of the CO A1Π-X1Σ+ system have been photographed at high resolution in emission and absorption. The perturbation analysis of the A1Π, v = 0 level has been extended up to J = 75. Bands of intercombination systems, namely (1-v″) of the e3Σ−-X1Σ+ and (4-v″) of the d3Δ-X1Σ+ bands, have been observed for J ≤ 40 because of intensity borrowing from the main A-X system. Above J ⋍ 40 , perturbations arising from a′3Σ+, v = 9; I1Σ−, v = 1; and e3Σ−, v = 2 are responsible for the irregular Λ-type doubling at the A1Π, v = 0 level. Accurate deperturbed constants for the A1Π state and the perturbing levels have been obtained; perturbation parameters and mixing coefficients which characterize each perturbation have been derived; and term values for the main and extra levels are presented.

Emission spectra obtained in the 1550–1650 A region with a 10-m vuv spectrograph are conclusively assigned to the N 2 2+ ion. The 1589-A band, previously observed by Carroll, and a new band of the same system, have been rotationally analyzed. Ab-initio calculations have been performed which support the assignment of these two bands to the D 1 Σ u + - X 1 Σ g + system. The calculations also explain the observed breaking-off points in the branch structure as well as weakening and broadening of the other expected bands. These phenomena arise from electron configuration changes and perturbation effects in the ground state.

Five new emission bands have been analyzed at high resolution in the short wavelength range of the vacuum ultraviolet emission spectrum of molecular nitrogen. Four of them are assigned to the band system c′ 6 1 Σ u + (v′ = 0) → X 1 Σ g + (v″ = 0–3) . The fifth band, at 82.6562 nm, corresponds to a transition from a highly excited state which was not known before but which is presently detected also in absorption at high resolution in 14N2 and 15N2. The new state is of 1Σu+ symmetry and is identified as the first term of a Rydberg series converging toward the A2Πu state of N2+. Both excited electronic states lie above the limit of dissociation of N2 into N( 2 D) + N( 2 D) . They are the only states in this energy range which are not totally predissociated.

The vacuum ultra-violet (V.U.V.) emission spectrum of N2, produced by a low pressure discharge, is investigated at high resolution from 107.8 nm down to 85 nm. Tens of newly observed bands show up at short wavelength. Cross-examination of emission and absorption spectra reveals that most of apparent intensity anomalies in rotational structures arise from radiation trapping, by accidental coincidence of an emission line with an absorption line from another band. It is concluded that very few anomalies can be ascribed to predissociation.